"Koko made da." means "Up until here." or "Until here only." literally in English.
"This is it." is a colloquial expression, and much like any other colloquial speech of any other language, some terms don't really mean what they literally mean, it has a hidden meaning.
"This is it." can mean several things depending on the situation.
For example, your long journey finally ends when you find that thing you're searching for. As an expression, you can say "This is it!!". With that meaning literally.
Also, it can be used to express a limit, like the on you said. For example, someone drives you along an unknown place, and he reaches his limit on where he can take you. He can say, "This is it.", meaning that he can't go further than this.
Another meaning would be when a long awaited/prepared for event comes along. For example, you're playing Pokemon and you've been training night and day to defeat the Unova region. When you reach the champion, you'd say BEFORE "This is it!" as an expression like "the time has finally come" or something along those words.
Meaning, "This is it" has different meaning depending on the situation, and when you translate any language, you don't only have to look at the text, but the situation as well.
"Koko made" is "Until here."
"da" is a copula.
"Koko made da." means "Up until here.", somewhat equivalent to "This is it.".
In that anime you were watching/video you were playing, what was the situation when the character said "Koko made da."?. Maybe that was what you were missing. The situation.
Hope I helped!